A hydraulic braking device is known in the art, according to which magnetic viscous fluid is filled in a fluid chamber formed in a housing and a brake rotating member is rotatably accommodated in the fluid chamber so that the brake rotating member is in contact with the magnetic viscous fluid. Viscosity of the magnetic viscous fluid is controlled (changed) when density of magnetic flux passing through the magnetic viscous fluid is changed. In the hydraulic braking device of this kind, it is possible to apply braking torque to the brake rotating member with a relatively small amount of electric power. Therefore, the hydraulic braking device is preferably applied to a valve timing adjusting apparatus for an internal combustion engine, according to which a relative phase (an engine operational phase) between a crankshaft and a cam shaft for deciding a valve timing (a valve opening and/or closing timing of an intake and/or an exhaust valve) is adjusted depending on a degree of the braking torque.
According to the hydraulic braking device known in the art, for example, as disclosed in Japanese Patent Publication No. 2010-121614 (A), a brake rotating member is rotatably accommodated in a fluid chamber of a housing, wherein a brake shaft extends in an inside of the housing and outwardly extends from the housing. A gap between the brake shaft and the housing is sealed by a sealing device. More in detail, a permanent magnet and a magnetic-flux guiding member of the sealing device are provided so as to surround the brake shaft, so that magnetic flux of the permanent magnet is guided from the magnetic-flux guiding member to the brake shaft via a sealing gap, which is formed between the magnetic-flux guiding member and the brake shaft and communicated to the fluid chamber. Then, magnetic viscous fluid, which flows from the fluid chamber into the sealing gap, receives the magnetic flux and thereby viscosity of the magnetic viscous fluid is increased. The magnetic viscous fluid is trapped in a film-like condition. More exactly, magnetic particles contained in the magnetic viscous fluid are trapped by the magnetic flux to form sealing films in the sealing gap.
The sealing films formed in the sealing gap restrict flow of the magnetic viscous fluid in an axial direction of the brake shaft from a housing-inner side to a housing-outer side. Namely, a so-called self-sealing function is brought out by the magnetic viscous fluid itself in the sealing gap. Leakage of the magnetic viscous fluid from the fluid chamber can be suppressed and thereby change of braking characteristic due to the leakage of the magnetic viscous fluid can be suppressed. In addition, when the leakage of the magnetic viscous fluid is suppressed by the sealing films, frictional resistance to be applied to the brake shaft can be reduced. Durability of the hydraulic braking device is thereby increased.
According to the above hydraulic braking device (JP No. 2010-121614), however, the braking characteristic may be changed due to the following factors. In the magnetic viscous fluid, the magnetic particles are dispersed in base fluid made of non-magnetic liquid. The non-magnetic base fluid does not receive action of the magnetic flux in the sealing gap but receives fluid pressure which is increased in the fluid chamber in accordance with increase of temperature. The base fluid is likely to flow in a direction to the housing-outer side. The leakage of the base fluid may not only cause change of property for the magnetic viscous fluid but also facilitate such change of property for the magnetic viscous fluid, because the leakage of the magnetic viscous fluid catches up the magnetic particles contained in the magnetic viscous fluid. The change of property for the magnetic viscous fluid decreases the self-sealing function in the sealing gap, which may indirectly or directly cause change of the braking characteristic, because the decrease of the self-sealing function in the sealing gap may influence the change of property for the magnetic viscous fluid in the fluid chamber.
A well-known mechanical sealing structure may be further used, in addition to the structure for the self-sealing function, so as to restrict the flow of the magnetic particles in the sealing gap from the housing-inner side to the housing-outer side. A magnetic fluid may be used as functional fluid in place of the magnetic viscous fluid, in which the magnetic particles are dispersed in the base fluid. However, in each case, it is necessary to form the sealing gap in order to improve durability. It is, therefore, still a problem that the base fluid may flow out through the sealing gap and thereby the property of the magnetic viscous fluid (or other functional fluid) may be changed. In other words, the braking characteristic of the hydraulic braking device may be changed.
Furthermore, according to the above hydraulic braking device (JP No. 2010-121614), it may have the following problem. When the magnetic particles contained in the magnetic viscous fluid are collected at such an area close to an inner peripheral portion of the magnetic-flux guiding member, the magnetic particles trapped by the magnetic flux in the sealing gap may be pushed out by such collected magnetic particles. Then, the magnetic particles may leak from the sealing gap to the housing-outer side.
More in detail, in the hydraulic braking device of the above prior art (JP No. 2010-121614), the magnetic-flux guiding member is partly covered by the magnetic shielding member. In other words, an axial end surface of the inner peripheral portion of the magnetic-flux guiding member is exposed to the fluid chamber via the communication gap. The magnetic flux may be leaked from such exposed portion of the magnetic-flux guiding member and thereby the magnetic particles contained in the magnetic viscous fluid may be collected at such a portion close to the sealing gap. The collected magnetic particles are magnetically attracted by the magnetic flux into the sealing gap. Then, the magnetic particles having been trapped by the magnetic flux in the sealing gap (for the purpose of forming the sealing films) may be pushed out from the sealing gap by such additionally attracted magnetic particles. Such leakage of the magnetic particles may decrease the self-sealing function, to thereby cause the variation of the braking characteristic for the hydraulic braking device.
According to another hydraulic braking device known in the art, for example, as disclosed in Japanese Patent Publication No. 2011-256838 (A), a gap between a brake shaft of a brake rotating member (which extends in an inside of a housing and outwardly extends from the housing) and the housing is sealed by a sealing structure. More exactly, a sealing gap, which is communicated to a fluid chamber, is formed between a magnetic screw portion of a male screw shape and a magnetic-flux guiding member. The magnetic screw portion is formed by a spiral projection, which is projected from the brake shaft in a radial outward direction. The magnetic-flux guiding member surrounds an outer periphery of the brake shaft, namely the magnetic screw portion. Since magnetic flux is guided between the magnetic-flux guiding member and the magnetic screw portion via the sealing gap, the magnetic viscous fluid is withdrawn from the fluid chamber into the sealing gap and viscosity of the magnetic viscous fluid is increased to thereby form sealing films. A self-sealing function is brought out, according to which leakage of the magnetic viscous fluid from the sealing gap to a housing-outer side is suppressed by such sealing films, namely by the magnetic viscous fluid itself.
According to the sealing structure of the above prior art (JP No. 2011-256838), the magnetic screw portion is so formed that the spiral projection comes away in the axial direction of the brake shaft from a housing-inner side of the brake shaft to a housing-outer side when tracing the spiral projection in a shaft rotating direction. Moment in a direction to the housing-inner side is given to the magnetic viscous fluid. The above moment is generated by visco-seal function (that is, a labyrinth-sealing effect of a screw-rotation type) based on hydro-dynamic effect for pumping up the magnetic viscous fluid by repeating compression and expansion and based on viscous effect by increase of viscosity of the magnetic viscous fluid. According to the visco-seal function, even when non-magnetic liquid of the magnetic viscous fluid is separated from magnetic particles, which are contained in the magnetic viscous fluid and trapped by the magnetic flux in the sealing gap, and has flown out from the sealing gap in the housing-outer side, such non-magnetic liquid is pushed back in the housing-inner side to the sealing gap.
As above, according to the sealing structure of the prior art (JP No. 2011-256838), the self-sealing function as well as the visco-seal function are brought out, so that variation of braking characteristic of the hydraulic braking device caused by possible leakage of the magnetic viscous fluid can be suppressed.
According to the sealing structure of the above prior art (JP No. 2011-256838), the non-magnetic liquid of the magnetic viscous fluid can be pushed back in the housing-inner direction by the visco-seal function when the brake rotating member is rotating. However, when the brake rotating member is not rotated, hydro-dynamic effect of the visco-seal function cannot be brought out. When the non-magnetic liquid is separated from the magnetic viscous fluid during non-operation (no-rotation) of the brake rotating member, the non-magnetic liquid may leak out of the sealing gap. Then, variation of braking characteristic may occur in the hydraulic braking device.